The present invention is related to circuits and methods for compensating for temperature changes in resistive circuits, and more specifically to a method and circuit for tuning an equivalent resistor in a filter so that the filter is insensitive to temperature changes.
Various types of electronic components require resistive circuits that are relatively insensitive to temperature. For example, filters for communication devices, such as for baseband signal and intermediate frequency filtering, require very accurate frequency response, but include resistors that have temperature sensitive resistance values. As the temperature changes, the resistance changes, thereby causing the filter to drift from a desired frequency.
Various methods for tuning a filter to the proper frequency in a communication device have been devised, but performance has limited their use. For example, filter tuning may be accomplished with a phase locked loop and a voltage controlled oscillator. However, these components require space, and in some filters, especially monolithic filters, space is expensive and the use of components that require space is desirably avoided. Further, a voltage controlled oscillator in a monolithic filter may increase noise and degrade performance.
In the present invention, a resistor in a filter is replaced with an equivalent resistor that includes a MOSFET that is operated in its linear region. The resistance of the equivalent resistor may be varied (that is, the filter may be tuned) by varying the voltage applied to the MOSFET gate to compensate for temperature changes. The mere use of equivalent resistors with MOSFETs does not necessarily solve the problems of the prior art, because a method of providing the correct voltage to the MOSFET gate to compensate for temperature changes must still be provided.
The prior art offers a solution that is improved upon in the present invention. With reference to FIG. 1, an equivalent resistor 10 with a MOSFET 12 may have a MOSFET gate voltage provided by an amplifier 14. Amplifier 14 has a reference voltage input 16 and a second input 18 that is a voltage across a second equivalent resistor 20 when a temperature insensitive reference current Iref is provided to second equivalent resistor 20 from current source 22. Second equivalent resistor 20 includes MOSFET 24 having a common gate with MOSFET 12 in first equivalent resistor 10. In operation, amplifier 14 senses a difference between a constant Vref and the changing voltage across second equivalent resistor 20. Amplifier 14 output thereby adjusts the voltage to the common gate to vary the resistance in first equivalent resistor 10 to thereby tune a filter that includes first equivalent resistor 10. See, for example, U.S. Pat. No. 5,107,199 issued Apr. 21, 1992 to Vo, et al.
The method of using a temperature insensitive Iref suggested by Vo, et al. removes some of the temperature-induced resistance variation from first equivalent resistor 20, and thus may be satisfactory for some applications. However, this method does not take advantage of further reductions in temperature-induced resistance variations that are available using the method and circuit of the present invention. In contrast to Vo, et al., the present invention uses a current source that is temperature sensitive so that a current used to generate inputs to the amplifier varies, preferably in inverse proportion to the conduction parameter, k (i.e., .mu.C.sub.ox /2), of the MOSFET. As will be demonstrated, the use of a current source that is temperature sensitive reduces temperature-induced variations in resistance significantly beyond that available from the method of Vo, et al.
Accordingly, it is an object of the present invention to provide a novel method and circuit for compensating for temperature changes in a resistive circuit that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method and circuit for compensating for temperature changes in a resistive circuit in which a current source provides inputs to a feedback amplifier that vary with temperature so that an output from the amplifier compensates for temperature changes.
It is yet another object of the present invention to provide a novel method and circuit for compensating for temperature changes in which a current source provides an input current for a feedback amplifier that varies in inverse proportion to a conduction parameter, k, of a MOSFET in the circuit.
It is still another object of the present invention to provide a novel method and circuit that decreases the temperature induced variation of resistance of a resistive element by using a temperature dependent current source instead of a temperature independent current source to control a feedback amplifier that adjusts a current across the resistive element.
It is a further object of the present invention to provide a novel method and circuit for tuning a resistor in a filter so that the filter is insensitive to temperature changes.
It is yet a further object of the present invention to provide a novel method and circuit for tuning a resistor in a filter in which a first equivalent resistor that is not part of the filter is operated with a temperature dependent input current so that a second equivalent resistor that is connected to the first equivalent resistor and that is part of the filter is compensated for temperature changes.
It is still a further object of the present invention to provide a novel method and circuit for tuning a resistor in a filter in which a first equivalent resistor that is not part of the filter is operated with an input current that is inversely proportional to the conduction parameter, k, of a MOSFET in the equivalent resistor.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.